Ignore it. 90% of the time it's a fake email address, and most of the rest
of the time it's just a way to verify that your email address actually
works before they sell it to other spammers.

Personally, running my own domain I'm amazed by the amount of spam I bounce
for non-existent email addresses, and the number of mailing lists which
have non-existent unicorn.com addresses subscribed. This annoys me a lot
more than the spam I'm sent, particularly when it ends up in my postmaster
mailbox for no good reason.